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Well water smells like rotten eggs

Updated 2026-09-15

Hydrogen sulfide. Run the hot and cold taps separately before doing anything else, because if only the hot water smells, the water heater is manufacturing it and the repair costs a fraction of a treatment system.

Why
Water heaters use a sacrificial magnesium anode, and Minnesota's health department describes how that magnesium can supply electrons that aid the conversion of sulfate in the water into hydrogen sulfide gas.
Watch out
Whole-house treatment for a problem that lives in the water heater is an expensive way to solve nothing. The hot-versus-cold comparison takes a minute and eliminates the most common cause.

Where it shows up

  • Smell strongest in hot water
  • Smell after water has sat unused
  • Tarnished silver
  • Dark staining on fixtures

Most likely causes

In the order worth ruling out.

  1. 1

    Hydrogen sulfide in well water

    The rotten egg smell, and the one water problem people notice instantly. Your nose finds it at concentrations a lab would call trace, and the treatment depends entirely on how much there is.

  2. 2

    Coliform and E. coli in well water

    The one result that is about the well's physical condition rather than the water's chemistry. Bacteria in a well mean something is getting in, and disinfection treats the consequence rather than the cause.

Do this before you price anything

Run the cold tap for a minute and smell it. Then run the hot tap and smell that.

Hot only. The well is probably innocent. Water heaters contain a sacrificial anode rod, commonly magnesium, that corrodes so the tank does not. Minnesota’s health department sets out the mechanism: that magnesium can supply electrons which aid the conversion of sulfate, harmless in itself and often present in well water, into hydrogen sulfide.

The tank is producing the smell out of something that arrived odourless.

Both hot and cold. The source is upstream of the heater, in the well or the plumbing, and treatment is the conversation.

Cold only. Unusual; check again, since sulfide tends to be more noticeable in hot water where it comes out of solution readily.

This single comparison separates a repair costing tens of dollars from a treatment system costing thousands, and it is skipped constantly.

If the heater is the cause

The standard response is replacing the magnesium anode with an aluminium or powered one. Minnesota’s guidance also covers disinfecting and flushing the heater with a chlorine bleach solution, because sulfur bacteria establish themselves in the tank and will re-seed a new anode.

Removing the anode altogether will stop the smell. It will also let the tank corrode considerably faster and usually voids the warranty, so it is a poor trade for the sake of a part that costs very little.

On a heater more than about ten years old, weigh this against replacement, since the anode is not the only thing at the end of its life.

If the well is the cause

Concentration decides the technology, and the Minnesota figures are the clearest published guidance available:

  • Below 1 mg/L, activated carbon handles it.
  • Up to about 6 mg/L, oxidising media filtration, which usually means an air-injection or greensand filter.
  • Above 6 mg/L, continuous chlorination or ozonation followed by filtration.

Sulfide and iron frequently occur together, because the same conditions produce both. Where the report shows both, one oxidising filter generally handles them together, which makes the combined problem cheaper to solve than two separate ones would suggest.

If the smell varies rather than holding steady, the likely cause is sulfur bacteria rather than geology, and the useful first step is shock chlorinating the well itself rather than treating what comes out of it indefinitely.

Read next

  • Symptom

    Orange or rust stains from well water

    Iron, in essentially every case. The useful question is not whether it is iron but which form, because dissolved iron and already-oxidised iron need different equipment.

  • Comparison

    Air injection vs chemical injection

    Air injection is the default and the right answer for most well iron. Chemical injection is for the specific cases air cannot reach: sulfide above about 6 ppm, very heavy iron, organic iron and iron bacteria.

  • Comparison

    UV vs chlorination for well water

    UV where the water is clear and the contamination is at the source. Chlorination where the water is not clear, where the plumbing itself is suspect, or where iron and sulfide need oxidising anyway.

  • Symptom

    Black stains or black specks in well water

    Manganese, in most cases, and it turns up at a twentieth of the concentration iron needs to cause trouble. It is also the reason a new iron filter sometimes fixes the orange staining and leaves the black staining exactly as it was.

  • Symptom

    Slime or sludge in well water

    Iron bacteria, most likely. They are living organisms that metabolise iron and produce a slime as they go, and that slime is why this problem does not respond to the equipment that handles ordinary iron.

  • Contaminant

    Nitrate in well water

    A genuine health standard rather than a staining threshold, and the population at risk is infants under six months. It is also a signal about the well itself, because nitrate is not naturally abundant in deep groundwater.

Not sure what is in your water yet?See which tests to order, thenplan the treatment from your results.

Questions

Only the hot water smells. What is the fix?
Replacing the magnesium anode rod with an aluminium or powered one, usually alongside flushing and disinfecting the tank. Removing the anode entirely will also stop the smell, at the cost of the heater corroding faster, and it generally voids the warranty, so replacement rather than removal is the advice.
Both hot and cold smell. What now?
The source is the well or the plumbing, and concentration decides the treatment. Minnesota's health department puts activated carbon as effective below 1 mg/L and oxidising media filtration as effective up to about 6 mg/L. Above that, continuous chlorination or ozonation with filtration is the answer.
The smell comes and goes. Does that mean anything?
It is a genuine clue. Sulfide from geology tends to be steady. Sulfide produced by sulfur bacteria varies with conditions and often worsens after the water has been standing, which points toward shock chlorinating the well rather than filtering its output forever.
Is it dangerous?
In water at the concentrations wells produce it is a nuisance and a corrosion problem rather than a health hazard, and there is no federal standard for it. As a gas accumulating in an enclosed space it is dangerous, which matters for well pits and confined spaces, not for a kitchen tap.

Sources